1. Field of the Invention
The present invention relates to an optical fiber with a built-in grating and an optical fiber used for the same.
2. Description of the Related Art
The structure of the fiber grating may be considered to be an array of transverse planes having a constant refractive index, i.e., equal refractive index planes, which are arranged at regular intervals in an optical fiber. An interval of the equal refractive index planes is a lattice interval, that is, a cycle in the fiber grating. The fiber grating reflects light having a limited wavelength around Bragg wavelength defined by the cycle and refractive index. The fiber grating can be formed by applying light, such as ultraviolet light, which induces a change in the refractive index, onto a photo-sensitive optical fiber, e.g., an optical fiber formed of silica glass to which germanium oxide (GeO2) has been added. An optical fiber with a fiber grating is used as a gain equalizer of a multiplexer/demultiplexer or an optical amplifier in a wavelength multiplex transmission system. The fiber grating used as the gain equalizer induces the coupling between the fundamental mode and a cladding mode of an optical fiber, causing strong power migration from the fundamental mode to the cladding mode. As a result, the fiber grating attenuates the intensity of light propagating in the fundamental mode over a certain wavelength band, functioning as a loss filter.
The fiber grating working as a loss filter is required to precisely control a loss spectrum. A narrower band of the loss filter is preferred because gain equalization with less excess loss can be achieved.